JP5623241B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and in particular, has a positive electrode active material in which fine particles made of a rare earth element compound adhere to the particle surface, and is excellent in high-temperature cycle characteristics even when the charging voltage is high, and battery swelling is suppressed. The present invention also relates to a non-aqueous electrolyte secondary battery.

  It has high energy density as a drive power source for portable electronic devices such as today's mobile phones, portable personal computers and portable music players, and also as a power source for hybrid electric vehicles (HEV) and electric vehicles (EV). However, non-aqueous electrolyte secondary batteries represented by high-capacity lithium ion secondary batteries are widely used.

The positive electrode active material of these nonaqueous electrolyte secondary batteries is represented by LiMO ₂ (where M is at least one of Co, Ni, and Mn) capable of reversibly occluding and releasing lithium ions. lithium transition metal composite oxide to be, _{namely, LiCoO 2, LiNiO 2, LiNi} y Co 1-y O 2 (y = 0.01~0.99), LiMnO 2, LiCo x Mn y Ni z O 2 (x + y + z = 1) and, like LiMn ₂ O ₄ or LiFePO ₄ is used as a mixture of one kind alone or in combination. In addition, as the negative electrode active material, a carbon material such as graphite or a material alloyed with lithium such as Si or Sn is used.

  Among these, since various battery characteristics are particularly excellent with respect to others, lithium cobalt composite oxides and heterogeneous metal element-added lithium cobalt composite oxides are often used. However, cobalt is expensive and has a small abundance as a resource. Therefore, in order to continue using these lithium cobalt composite oxides and lithium cobalt composite oxides containing different metal elements as positive electrode active materials for non-aqueous electrolyte secondary batteries, further enhancement of the performance of non-aqueous electrolyte secondary batteries is desired. ing.

In particular, non-aqueous electrolyte secondary batteries are required to have higher capacities due to demands for increased power consumption and long-term driving of HEVs and EVs with enhancement of entertainment functions such as video playback and game functions in recent mobile information terminals. Has been. As a measure to increase the capacity of non-aqueous electrolyte secondary batteries,
(1) Increase the capacity of the active material,
(2) Increase the charging voltage,
(3) Increase the filling amount of the active material to increase the filling density,
Such a method is conceivable.

  However, especially when the charging voltage is increased, specifically, when the charging potential of the positive electrode is higher than 4.3 V on the basis of lithium, the crystal structure of the positive electrode active material becomes unstable, and oxygen molecules or oxygen radicals are generated. It tends to occur. As a result, oxidative decomposition of the electrolytic solution is likely to occur, and gas generation due to oxidative decomposition of the electrolytic solution, polarization resistance due to deposition of decomposition products, and deterioration of the positive electrode material due to progress of dissolution of the positive electrode active material may occur. There is a problem that the cycle characteristics are deteriorated and the battery thickness is increased due to gas generation.

  As a technique for preventing oxidative decomposition of an electrolytic solution, for example, Patent Document 1 listed below discloses a positive electrode active material in which rare earth hydroxide and oxyhydroxide fine particles are dispersed on the surface of a lithium transition metal composite oxide. It has been shown that the use of can suppress the electrolyte decomposition reaction when charged and stored at a high temperature, and can suppress battery swelling.

  Patent Document 1: WO2010 / 004973

  However, when the charge voltage is increased, when a positive electrode active material whose particle surface is coated with a rare earth element compound as described in Patent Document 1 is used for the positive electrode of the nonaqueous electrolyte secondary battery, the capacity decreases during high-temperature charge storage. Although the battery expansion is sufficiently suppressed, the problem that the capacity is reduced and the battery expansion during the high temperature cycle is large has been found. This is because the surface of the positive electrode active material particles is coated with a rare earth element compound, so that deterioration due to charge / discharge is difficult to progress, and the crystal structure associated with deposition and charge / discharge of electrolytic solution reduction decomposition products. The material deterioration of the negative electrode, where brittle fracture of the metal proceeds at a constant speed, proceeds relatively quickly. As a result, when the negative electrode charge capacity falls below the positive electrode charge capacity, lithium metal is deposited on the negative electrode. This is because the deposited lithium metal is very reactive and accelerates the reductive decomposition of the electrolytic solution.

  On the other hand, when a positive electrode active material whose surface is not coated with a rare earth element is used, if the charging voltage is increased, as described above, the oxidative decomposition of the electrolyte solution at the positive electrode is not suppressed. There is a problem of increase in battery thickness due to generation.

  The present invention has been made to solve the above-described problems of the prior art, and even when the charging potential is increased, the oxidative decomposition of the nonaqueous electrolytic solution at the positive electrode is suppressed, An object of the present invention is to provide a nonaqueous electrolyte secondary battery in which the deposition of lithium metal on the negative electrode is suppressed, the cycle characteristics at high temperatures are excellent, and the gas generation is suppressed.

  In order to achieve the above object, a nonaqueous electrolyte secondary battery of the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and a nonaqueous electrolyte. The positive electrode active material is at least one of lithium cobalt oxide and nickel / cobalt / manganese manganate, and is composed of at least one of a rare earth element hydroxide and an oxyhydroxide on the particle surface. A positive electrode active material A to which fine particles are attached; and a positive electrode active material B made of at least one of lithium molybdate and lithium tungstate. It is 0.01 mass% or more and 2.0 mass% or less with respect to the substance A, It is characterized by the above-mentioned.

  According to the nonaqueous electrolyte secondary battery of the present invention, a nonaqueous electrolyte secondary battery having excellent high-temperature cycle characteristics and well-suppressed battery swelling can be obtained even when the charging voltage is increased.

  By using the positive electrode active material A whose particle surface is coated with a rare earth element compound, deterioration of the positive electrode material is suppressed. However, when the positive electrode active material A is used alone as the positive electrode active material, when the charging voltage is increased, the material deterioration of the negative electrode becomes relatively faster than the deterioration of the positive electrode material, so that the charge capacity of the negative electrode is increased. Lithium metal begins to be deposited on the negative electrode when the charge capacity falls below the charge capacity of the battery, and the reductive decomposition of the electrolyte solution is accelerated by the deposited lithium. As a result, the battery capacity deteriorates and the battery swells. .

  Here, when a small amount of the positive electrode active material B is contained in the positive electrode active material A, the positive electrode active material B has a feature that the transition metal is easily dissolved along with charge / discharge, that is, the cycle deterioration is quick. The positive electrode material is appropriately deteriorated. As a result, even when the charging voltage is increased, the balance between the deterioration rate of the positive electrode material and the deterioration rate of the negative electrode material is improved, and the oxidative decomposition of the electrolyte solution at the positive electrode is sufficiently suppressed, while the lithium metal is deposited at the negative electrode. As a result, high-temperature cycle characteristics are improved, and battery swelling is also suppressed.

  If the content of the positive electrode active material B is too small or too large, the balance between the deterioration rate of the positive electrode and the deterioration rate of the negative electrode is deteriorated, and cycle characteristics and battery swelling are deteriorated. Therefore, it is necessary that the content be 0.01% by mass or more and less than 5.0% by mass, and 0.01% by mass or more and 2.0% by mass or less with respect to the positive electrode active material mixture. More preferably, it is 0.01% by mass or more and 1.0% by mass or less.

  The fine particles comprising at least one of rare earth element hydroxide and oxyhydroxide in the positive electrode active material A are, for example, particles comprising at least one of lithium cobaltate and nickel / cobalt / lithium manganate. The rare earth element hydroxide is precipitated in a solution in which is dispersed, and the rare earth element hydroxide is adhered to the surface of particles of at least one of lithium cobaltate and nickel / cobalt / lithium manganate. It can be obtained by a manufacturing method including a step and a step of performing a heat treatment.

  In general, the heat treatment temperature is preferably in the range of 80 to 600 ° C, and more preferably in the range of 80 to 400 ° C. When the temperature of the heat treatment is higher than 600 ° C., some of the fine particles of the rare earth element compound adhering to the surface diffuse into the active material, and the initial charge / discharge efficiency decreases. Therefore, in order to obtain an active material having a high capacity and a surface in which a rare earth element compound is adhered more selectively, the heat treatment temperature is preferably 600 ° C. or lower. Further, since the hydroxide is in the form of hydroxide, oxyhydroxide, oxide, etc. by heat treatment, the rare earth element compound adhering to the surface of the positive electrode active material in the present invention is hydroxide, oxyhydroxide. It adheres in the form of objects, oxides, etc. Here, when heat-treated at 400 ° C. or lower, it is mainly in the state of hydroxide or oxyhydroxide. The heat treatment time is preferably about 3 to 7 hours.

  The rare earth elements that can be used in the positive electrode active material A of the present invention include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium ( Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) can be used. Promethium (Pm) is also a kind of rare earth element, but it is a radioisotope and a stable isotope cannot be obtained.

  In addition to the positive electrode active material, the positive electrode in the nonaqueous electrolyte secondary battery of the present invention may contain a conductive agent or a binder that has been conventionally used. Moreover, what consists of aluminum or an aluminum alloy can be used as a core of a positive electrode. Furthermore, as the negative electrode active material, carbon materials such as graphite and coke, metals that can be alloyed with lithium such as tin oxide, metallic lithium, and silicon, and alloys thereof can be used. What consists of copper or a copper alloy can be used.

  Nonaqueous solvents that can be used in the nonaqueous electrolyte secondary battery of the present invention include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and fluorinated cyclic carbonates. Esters, cyclic carboxylic acid esters such as γ-butyrolactone (BL) and γ-valerolactone (VL), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), dibutyl Chain carbonates such as carbonate (DBC), fluorinated chain carbonates, chain carboxylates such as methyl pivalate, ethyl pivalate, methyl isobutyrate, methyl propionate, N, N′— Dimethylformamide, N-me Amide compounds such as oxazolidinone, sulfur compounds such as sulfolane, etc. ambient temperature molten salt such as tetrafluoroboric acid 1-ethyl-3-methylimidazolium can be exemplified. It is desirable to use a mixture of two or more of these. Among these, cyclic carbonates and chain carbonates having a particularly high dielectric constant and a high ionic conductivity of the nonaqueous electrolytic solution are preferable.

  In addition, as a separator used in the nonaqueous electrolyte secondary battery of the present invention, a separator made of a microporous film formed from a polyolefin material such as polypropylene or polyethylene can be selected. In order to ensure the shutdown response of the separator, a resin having a low melting point may be mixed, and further, a laminate with a high melting point resin or a resin carrying inorganic particles may be used to obtain heat resistance.

  In the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention, as a compound for stabilizing the electrode, vinylene carbonate (VC), vinyl ethyl carbonate (VEC), succinic anhydride (SUCAH) , Maleic anhydride (MAAH), glycolic anhydride, ethylene sulfite (ES), divinyl sulfone (VS), vinyl acetate (VA), vinyl pivalate (VP), catechol carbonate, biphenyl (BP), etc. Good. Two or more of these compounds can be appropriately mixed and used.

In addition, as the electrolyte salt dissolved in the non-aqueous solvent used in the non-aqueous electrolyte secondary battery of the present invention, a lithium salt generally used as an electrolyte salt in the non-aqueous electrolyte secondary battery can be used. Such lithium salts include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , and mixtures thereof Illustrated. Among these, LiPF ₆ (lithium hexafluorophosphate) is particularly preferable. The amount of electrolyte salt dissolved in the non-aqueous solvent is preferably 0.8 to 1.5 mol / L.

  Furthermore, in the non-aqueous electrolyte secondary battery of the present invention, the non-aqueous electrolyte may be not only liquid but also gelled.

  In the nonaqueous electrolyte secondary battery of the present invention, it is preferable that the average particle diameter of the fine particles comprising at least one of the rare earth element hydroxide and oxyhydroxide is 100 nm or less.

  In the non-aqueous electrolyte secondary battery of the present invention, when the average particle size of the fine particles comprising at least one kind of rare earth element hydroxide and oxyhydroxide becomes larger and exceeds 100 nm, the lithium transition metal composite oxide It becomes difficult to adhere to the surface of the film, and it is difficult to achieve the desired effect. In addition, when the average particle size of these fine particles becomes small, it tends to adhere to the surface of the lithium transition metal composite oxide, but since the surface of the lithium transition metal composite oxide is densely coated, the lithium transition metal composite oxide The characteristic as a positive electrode active material of an oxide comes to fall. The average particle diameter of the fine particles comprising at least one of a more preferable rare earth element hydroxide and oxyhydroxide is in the range of 1 to 100 nm, and more preferably in the range of 10 to 100 nm.

  In the nonaqueous electrolyte secondary battery of the present invention, the amount of fine particles comprising at least one of the rare earth element hydroxide and the oxyhydroxide in the positive electrode active material A is the lithium cobalt oxide. And it is preferable that they are 0.01 mol% or more and 0.3 mol% or less with respect to at least 1 sort (s) of nickel * cobalt * lithium manganate.

  In the nonaqueous electrolyte secondary battery of the present invention, the adhesion amount of the fine particles comprising at least one kind of rare earth element hydroxide and oxyhydroxide in the positive electrode active material A is less than 0.01 mol%. In some cases, the gas generation suppression effect during the high-temperature cycle cannot be sufficiently obtained. Further, if the adhesion amount of the fine particles exceeds 0.3 mol%, the durability of the positive electrode active material increases, and it becomes difficult to maintain the positive / negative electrode discharge performance balance during a long-term cycle. The adhesion amount of the fine particles comprising at least one kind of rare earth element hydroxide and oxyhydroxide is the adhesion amount to the positive electrode active material A, for example, the adhesion amount of the fine particles is 0.1 mol%. In this case, 0.001 mol of fine particles are attached to 1 mol of the positive electrode active material A to which fine particles are attached. The amount of fine particles attached is a value in terms of rare earth elements.

  In the nonaqueous electrolyte secondary battery of the present invention, the charging potential of the positive electrode can be 4.35 V or more and 4.6 V or less on the basis of lithium.

  The charging potential of lithium cobaltate, which is widely used as a positive electrode active material, is 4.3 V with respect to lithium. In the nonaqueous electrolyte secondary battery of the present invention, the charge potential of the positive electrode can be set to 4.35 V or more on the basis of lithium, so that it is used to a value closer to the theoretical capacity than when lithium cobaltate is used as the positive electrode active material. In addition, it is possible to increase the capacity and energy density of the battery. In addition, since the positive electrode active material is decomposed if the upper limit of the charging potential of the positive electrode becomes too high, the upper limit of the charging potential is preferably 4.6 V or less on the basis of lithium.

  In the nonaqueous electrolyte secondary battery of the present invention, the negative electrode active material is preferably made of graphite.

  When a negative electrode active material used for a non-aqueous electrolyte secondary battery is made of graphite, dendrite grows while having a discharge potential (0.1 V based on lithium) comparable to lithium metal or lithium alloy. In other words, the safety is high, the initial efficiency is excellent, the potential flatness is good, and the density is high, so that a high-capacity nonaqueous electrolyte secondary battery can be obtained.

  Hereinafter, the form for implementing this invention is demonstrated in detail using an Example and a comparative example. However, the following examples illustrate non-aqueous electrolyte secondary batteries for embodying the technical idea of the present invention, and are not intended to specify the present invention to these examples. The present invention can be equally applied to various modifications without departing from the technical idea shown in the claims.

[Example 1]
[Preparation of positive electrode plate]
<Preparation of positive electrode active material A>
As a starting material of the positive electrode active material A, lithium carbonate (Li ₂ CO ₃ ) was used as a lithium source, cobalt carbonate was calcined at 550 ° C. as a cobalt source, and tricobalt tetroxide obtained by a thermal decomposition reaction ( Co ₃ O ₄ ) was used. These were weighed so that the molar ratio of lithium to cobalt was 1: 1. Thereafter, the mixture was mixed in a mortar, and the mixture of lithium carbonate and tricobalt tetroxide was baked at 850 ° C. for 20 hours in an air atmosphere to obtain lithium cobaltate (LiCoO ₂ ) as the positive electrode active material a.

The lithium cobalt oxide as the positive electrode active material material a obtained as described above was pulverized to an average particle size of 15 μm with a mortar, and then 1000 g was added to 3 liters of pure water and stirred to disperse the lithium cobalt oxide particles. In this suspension, a predetermined rare earth nitrate hydrate was added in an amount of 0.1 mol% with respect to lithium cobaltate in terms of rare earth elements (in Example 1, erbium trinitrate · 5 water). A solution (4.53 g of Japanese (Er (NO ₃ ) ₃ .5H ₂ O)) was added as an aqueous solution. In addition, when adding the solution in which the hydrate of the rare earth nitrate is dissolved to the lithium cobalt oxide suspension, the pH of the suspension is adjusted to 9 by adding a 10% by mass sodium hydroxide aqueous solution. Kept.

The lithium cobalt oxide suspension added with the rare earth nitrate obtained as described above is subjected to suction filtration and washed with water to obtain a powder. The powder is then dried at 120 ° C. A rare earth hydroxide, that is, lithium cobaltate having erbium hydroxide uniformly adhered thereto was obtained. Subsequently, the lithium cobalt oxide having the rare earth hydroxide adhered to the particle surface is heat-treated at 300 ° C. for 5 hours in an air atmosphere, so that the lithium cobalt oxide having the erbium compound adhered to the particle surface (hereinafter referred to as “positive electrode active material a”). ₁₀ ”), and the positive electrode active material A according to Example 1 was obtained.

The above for the positive electrode active material a _10, was observed by a scanning electron microscope (SEM), the surface of the lithium cobalt oxide particles, adhered in a state of following the erbium compound average particle diameter 100nm is uniformly dispersed It was. Adhesion amount of ICP erbium compound (Inductively Coupled Plasma: inductive coupled plasma) results as measured by emission spectrometry, elemental erbium terms, a 0.1 mol% with respect to lithium cobalt oxide, when the positive electrode active material a ₁₀ produced It was confirmed that almost the whole amount of erbium added to was adhered to the cobalt acid particle surface.

<Preparation of positive electrode active material mixture slurry>
The positive electrode active material A obtained as described above and the lithium molybdate (Li ₂ MoO ₃ ) as the positive electrode active material B are 96 parts by mass as a predetermined ratio and the total amount of the positive electrode active material, that is, In addition to 95.95 parts by mass of the positive electrode active material a ₁₀ , 0.05 part by mass of lithium molybdate, and further 2 parts by mass of carbon powder as a conductive agent, polyvinylidene fluoride (PFdV) as a binder After mixing so that a powder might be 2 mass parts, the positive electrode active material mixture was prepared, this was mixed with the N-methylpyrrolidone (NMP) solution, and the positive electrode active material mixture slurry was prepared.

<Preparation of positive electrode plate>
This positive electrode active material mixture slurry is applied to both sides of an aluminum foil as a positive electrode core having a thickness of 15 μm and a length of 334 mm. The applied mass on one side is 21.2 mg / cm ² , and the applied part on one side is 277 mm, not applied. The coating was performed so that the portion was 57 mm, the coated portion on the other side was 208 mm, and the uncoated portion was 126 mm. Thereafter, the electrode plate was obtained by passing through a dryer and drying. Subsequently, the positive electrode plate concerning Example 1 was produced by compressing so that the thickness of a double-sided application part might be 132 micrometers using a compression roller.

  In Examples 2 to 11 and Comparative Examples 1 to 6, as shown in Table 1 and below, the type of the active material of the positive electrode active material A and the particle surface of the active material are attached to Example 1 with respect to Example 1. Positive electrode plates differing in the type and amount of rare earth elements, the type of positive electrode active material B, the mixing ratio of the positive electrode active material A and the positive electrode active material B, and the like were produced.

[Examples 2 to 5]
That is, in Examples 2 to 5, instead of the positive electrode active material a ₁₀ in Example 1, ytterbium compound (Example 2), terbium compound (Example 3), holmium compound ( Example 4), except that the positive electrode active materials a ₂ , a ₃ , a ₄ , and a ₅ to which the lutetium compound (Example 5) was attached were used as the positive electrode active material A in the same manner as in Example 1. A positive electrode plate was prepared.

In attaching the rare earth element compound, a positive electrode active material A was obtained in the same manner as in Example 1 except that the rare earth nitrate oxide dissolved in the aqueous rare earth element nitrate solution added to the lithium cobaltate suspension was changed. That is, in Example 2, 4.22 g of ytterbium trinitrate trihydrate (Yb (NO ₃ ) ₃ .3H ₂ O), and in Example 3, terbium trinitrate hexahydrate (Tb (NO ₃ ) ₃ 4.63 g of 6H ₂ O), 4.51 g of holmium trinitrate pentahydrate (Ho (NO ₃ ) ₃ · 5H ₂ O) in Example 4 and lutetium trinitrate trihydrate in Example 5 1000 g of lithium cobaltate as positive electrode active material a was added to 3 liters of pure water and stirred, with 4.24 g of each product (Lu (NO ₃ ) ₃ · 3H ₂ O)) dissolved therein By adding while adding a 10% by mass aqueous sodium hydroxide solution to the lithium cobaltate suspension so that the pH of the suspension is maintained at 9, a predetermined rare earth nitrate is obtained. To give the added lithium cobalt oxide suspension 0.1 mol% relative to lithium cobaltate in rare earth element conversion.

Next, the powder obtained after suction filtration and washing with water of the lithium cobalt oxide suspension to which the rare earth nitrate is added is dried at 120 ° C. so that a predetermined rare earth hydroxide is uniformly attached to the surface of the particles. Lithium cobaltate was obtained. Next, lithium cobalt oxide having a rare earth hydroxide adhering to the particle surface is heat-treated at 300 ° C. for 5 hours in an air atmosphere to obtain lithium cobalt oxide having a predetermined rare earth element compound adhering to the particle surface. the positive electrode active material a according to example 2-5, i.e., as the positive electrode active material _{a 2, a 3, a 4} , a 5, to produce such a positive electrode plate in example 2-5.

[Examples 6 and 7]
In Example 6 and 7, in place of the positive electrode active material a ₁₀ in the first embodiment, the deposition amount of the erbium compound adhered to the particle surface of the lithium cobalt oxide, the lithium cobalt oxide, 0.01 mol% (example 6), was 0.3 mol% (example 7), the positive electrode active material _{a 11, a 12,} except for using as the positive electrode active material a was prepared a positive electrode plate in the same manner as in example 1 .

  That is, while an aqueous solution in which 4.53 g of erbium trinitrate pentahydrate was dissolved in Example 1 was added, 0.45 g of erbium trinitrate pentahydrate (Example 6) to 13 .59 g (Example 7) By using the dissolved aqueous solution, the amount of erbium trinitrate hydrate added to the lithium cobaltate suspension was 0.01 mol% (based on erbium) with respect to lithium cobaltate. A positive electrode active material A was obtained in the same manner as in Example 1 except that it was changed to Example 6) to 0.3 mol% (Example 7), and positive electrode plates according to Examples 6 to 7 were produced.

[Examples 8 and 9 and Comparative Examples 1 to 4]
Moreover, in Examples 8 and 9 and Comparative Examples 3 and 4, Example 1 was different from Example 1 except that the mixing ratio of the positive electrode active material A and the positive electrode active material B was changed to prepare a positive electrode active material mixture slurry. A positive electrode plate was produced in the same manner. Further, in Comparative Examples 1 and 2, which do not apply together the positive electrode active material B when the positive electrode active material a not to adhere rare earth element: the cathode active in (positive electrode active material A 'Comparative Example 1) to a positive electrode active material a ₁₀ A positive electrode plate was prepared in the same manner as in Example 1 except that the material B not applied (Comparative Example 2) was used.

That is, the mixing ratio of the positive electrode active material _{a 10} and lithium molybdate was 95.95 parts by weight and 0.05 part by weight in Example 1, 95.99 parts by mass of 0.01 parts by weight (Example 8) 95.0 parts by mass and 1.0 parts by mass (Example 9) 95.995 parts by mass and 0.005 parts by mass (Comparative Example 3) 91.0 parts by mass and 5.0 parts by mass (Comparative Example 4) ), Or 96 parts by mass (only Comparative Example 1) or 96 parts by mass (only the positive electrode active material a ₁₀ ) of the positive electrode active material A ′ composed of only the positive electrode active material a to which no rare earth element is adhered ( For Comparative Example 2), 2 parts by mass of carbon powder as a conductive agent and 2 parts by mass of polyvinylidene fluoride powder as a binder were mixed to prepare a positive electrode active material mixture, which was then mixed with N-methylpyrrolidone. Positive electrode active material obtained by mixing with solution With slurry, Example 8, Example 9 was prepared according positive electrode plate in comparative example 1-4.

[Example 10]
In Example 10, a positive electrode plate was prepared in the same manner as in Example 1 except that lithium tungstate (Li ₂ WO ₃ ) was used as the positive electrode active material B instead of lithium molybdate in Example 1. Produced.

That is, 2 parts by mass of carbon powder and 2 parts by mass of polyvinylidene fluoride powder with respect to 96 parts by mass of the total amount of positive electrode active material including 95.95 parts by mass of positive electrode active material a ₁₀ and 0.05 parts by mass of lithium tungstate. Were mixed to prepare a positive electrode active material mixture, and then a positive electrode plate according to Example 10 was produced using a positive electrode active material mixture slurry obtained by mixing this with an N-methylpyrrolidone solution.

[Example 11]
In Example 11, a positive electrode active material that instead of the positive electrode active material _{a 10} in Example 1, nickel-cobalt-lithium manganate _{(LiNi 0.33 Co 0.34 Mn 0.33 O} 2) except that nickel, cobalt, and lithium manganate (hereinafter referred to as “positive electrode active material b ₁ ”) obtained by attaching an erbium compound to b and having an erbium compound attached to the particle surface is used as the positive electrode active material A. A positive electrode plate was produced in the same manner as in Example 1.

That is, the positive electrode plate according to Example 11 was produced as follows. In the production of nickel, cobalt, and lithium manganate as the positive electrode active material b, lithium hydroxide (LiOH.H ₂ O) is used as a starting material, and nickel, cobalt, and manganese sources are nickel. Nickel-cobalt-manganese composite hydroxide obtained by coprecipitation of a predetermined amount of cobalt and manganese was used. These were weighed so that the molar ratio of lithium to nickel / cobalt / manganese was 1: 1, mixed in a mortar, baked at 400 ° C. for 12 hours in an oxygen atmosphere, crushed in a mortar, and then in an oxygen atmosphere The nickel, cobalt, and lithium manganate were obtained by baking at 900 degreeC for 24 hours below.

  Nickel, cobalt, and lithium manganate as the positive electrode active material b obtained as described above were pulverized to an average particle size of 15 μm in a mortar, and then 1000 g was added to 3 liters of pure water and stirred. After preparing a suspension in which cobalt-lithium manganate particles were dispersed, an aqueous solution in which 4.60 g of erbium trinitrate pentahydrate was dissolved in this suspension was obtained in the same manner as in Example 1. While maintaining a pH of 9 at 10%, a 10% by mass aqueous sodium hydroxide solution was added together.

Subsequently, the nickel-cobalt-manganese manganate suspension added with erbium trinitrate obtained as described above was subjected to suction filtration and water washing in the same manner as in Example 1, and the obtained powder was washed at 120 ° C. After drying to obtain nickel / cobalt / lithium manganate in which erbium hydroxide is uniformly attached to the surface of the particles, heat treatment is performed at 300 ° C. for 5 hours in an air atmosphere, whereby the positive electrode active material b ₁ is obtained. Nickel / cobalt / lithium manganate having an erbium compound adhered to the particle surface was obtained and used as the positive electrode active material A according to Example 11.

In the preparation of the positive electrode active material mixture slurry, the mixing ratio of the positive electrode active material A and the positive electrode active material B is the same as in Example 1, that is, molybdic acid with respect to 95.95 parts by mass of the positive electrode active material b _1. A positive electrode active material mixture was prepared by adding 0.05 parts by mass of lithium, 2 parts by mass of carbon powder, and 2 parts by mass of polyvinylidene fluoride powder to prepare a positive electrode active material mixture, and then adding this to N-methylpyrrolidone. A positive electrode active material mixture slurry was prepared by mixing with a solution, and a positive electrode plate according to Example 11 was produced.

[Comparative Example 5]
In Comparative Example 5, when preparing the positive electrode active material mixture slurry in Example 1 was mixed with the positive electrode active material a ₁₀ as a positive electrode active material A, lithium molybdate as a positive electrode active material B In contrast, when preparing the positive electrode active material mixture slurry, lithium molybdate as the positive electrode active material B is not added. Instead, in the preparation process of the positive electrode active material A, a lithium source and a cobalt source are used. A cathode active material obtained by adhering an erbium compound to the particle surface of molybdenum-added lithium cobalt oxide (hereinafter referred to as “cathode active material c”) obtained by adding molybdenum to the mixture and firing. only material c ₁ to prepare a positive electrode plate used as a cathode active material.

  That is, in the positive electrode plate according to Comparative Example 5, instead of the positive electrode active material a in Comparative Example 1, a mixture of lithium carbonate and tricobalt tetroxide (a molar ratio of lithium and cobalt of 1: 1) is 95.95. After mixing part by mass with 0.05 part by mass of a mixture of lithium carbonate and molybdenum trioxide as a molybdenum source (a molar ratio of lithium and molybdenum of 2: 1), firing was performed at 850 ° C. for 20 hours in an air atmosphere. Except for using the positive electrode active material c obtained by doing this, it was produced in the same manner as in Comparative Example 1, and a specific production method will be described below.

  First, the positive electrode active material c obtained as described above was pulverized in a mortar to an average particle size of 15 μm, and then 1000 g was added to 3 liters of pure water and stirred to obtain a suspension in which the positive electrode active material c was dispersed. A liquid was prepared. To this suspension was added an aqueous solution in which 4.53 g of erbium trinitrate pentahydrate was dissolved, together with a 10% by weight aqueous sodium hydroxide solution so that the pH of the suspension was maintained at 9. Added.

Next, the positive electrode active material c and erbium trinitrate suspension obtained as described above were subjected to suction filtration and water washing in the same manner as in Example 1, and the obtained powder was dried at 120 ° C. The positive electrode active material c ₁ obtained by uniformly adhering erbium hydroxide to the surface of the particles and then heat-treating at 300 ° C. for 5 hours in an air atmosphere was used as the positive electrode active material A according to Comparative Example 5. .

Next, with respect to 96 parts by mass of the positive electrode active material c ₁ obtained as described above, in the same manner as in Comparative Example 1, the positive electrode active material B was not added, but 2 parts by mass of carbon powder as a conductive agent and binding were performed. Comparative Example Using a positive electrode active material mixture slurry obtained by mixing with 2 parts by weight of polyvinylidene fluoride powder as an agent to prepare a positive electrode active material mixture and then mixing it with an N-methylpyrrolidone solution The positive electrode plate concerning 5 was produced.

[Comparative Example 6]
Moreover, as a positive electrode plate of Comparative Example 6, a positive electrode active material made of nickel, cobalt, and lithium manganate (LiNi _0.33 Co _0.34 Mn _0.33 O ₂ ) produced in the same manner as in Example 11. 96 parts by mass of positive electrode active material b ₁ made of nickel, cobalt, and lithium manganate obtained by attaching an erbium compound to material b and made of nickel, cobalt, and lithium manganate with an erbium compound attached to the particle surface is used as positive electrode active material B. A positive electrode plate was produced in the same manner as in Example 11 except that was not added.

[Production of negative electrode plate]
An appropriate amount of 97.5 parts by mass of graphite as a negative electrode active material, 1.0 part by mass of carboxymethyl cellulose (CMC) as a thickener, and 1.5 parts by mass of styrene butadiene rubber (SBR) as a binder. Was mixed with water to obtain a negative electrode active material mixture slurry. This slurry was applied to both sides of a copper foil as a negative electrode core having a thickness of 10 μm and a length of 299 mm. The coated mass on one side was 11.3 mg / cm ² , the coated part on one side was 284 mm, the uncoated part was 15 mm, It applied by the doctor blade method so that the application part of one side might be 226 mm and an unapplication part might be 73 mm. Thereafter, the electrode plate was obtained by passing through a dryer and drying. Subsequently, the negative electrode plate used by each Example and a comparative example was produced by compressing so that the thickness of a double-sided application part might be set to 155 micrometers using a compression roller. Note that the potential of graphite during charging is about 0.1 V on the basis of Li. The positive electrode and negative electrode active material filling amount is such that the charge capacity ratio of the positive electrode to the negative electrode (negative electrode charge capacity / positive electrode charge capacity) is 1.0 to 1.1 at the potential of the positive electrode active material as a design standard. Adjusted.

[Preparation of electrolyte]
After dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / L in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 7 Then, 1% by mass of vinylene carbonate (VC) was added to prepare an electrolyte solution used in each example and comparative example.

[Production of flat wound electrode body]
The positive electrode plate and the negative electrode plate according to each example and comparative example produced as described above were welded with an aluminum lead wire on the positive electrode plate and a nickel lead wire on the negative electrode plate. After that, a spiral electrode body used in each example and comparative example was manufactured by winding it in a flat shape through a separator made of a polyethylene microporous film.

[Preparation of non-aqueous electrolyte battery]
The flat wound electrode body produced as described above was sealed in a laminate container, and the obtained electrolytic solution was injected in a glove box filled with Ar. Then, the non-aqueous electrolyte secondary battery (design capacity: 800 mAh) concerning each Example and a comparative example was produced by plugging the liquid injection port.

[High voltage high temperature cycle characteristics test]
About the nonaqueous electrolyte secondary battery concerning each Example and comparative example produced as mentioned above, the high voltage high temperature cycling characteristic test was done on condition of the following.
Charging: Constant current charging is performed until the battery voltage reaches 4.4 V (positive electrode potential is 4.5 V based on lithium) at a current of 0.5 It (400 mA), and then the current value is 40 at a constant voltage of 4.4 V. The battery was charged to 0.0 mA.
Discharge: Constant current discharge was performed until the battery voltage reached 3.0 V (positive electrode potential was 3.1 V based on lithium) at a current of 0.5 It (400 mA).
-Pause: The pause interval between charge and discharge and between discharge and charge was 10 minutes each.
-Environmental temperature: It implemented in the 45 degreeC thermostat.
Charging-resting-discharging-resting under the above conditions is one cycle of charging / discharging, the charging / discharging cycle is repeated 500 cycles, and the value obtained from the following calculation formula from the first discharging capacity and the 500th discharging capacity: Was determined as high voltage high temperature cycle characteristics (%).
High voltage high temperature cycle characteristics (%)
= (500th cycle discharge capacity / 1st cycle discharge capacity) × 100

In addition, before and after the cycle characteristic test, the battery thickness was measured for each of the examples and comparative examples, and the amount of increase in battery thickness due to 500 cycles of continuous charge and discharge was determined.
These results are summarized in Table 1.

  From the results shown in Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 to 11 have improved high-voltage and high-temperature cycle characteristics and good battery thickness increase compared to those of Comparative Examples 1 to 5. It can be seen that it is suppressed. The reason for this effect is considered as follows.

  That is, by comparing Comparative Example 1 and Comparative Example 2, the surface of lithium cobalt oxide particles as the positive electrode active material is coated with an erbium compound, so that high voltage and high temperature cycle characteristics are improved and battery thickness increase is reduced to some extent. I understand that

  This is because, in Comparative Example 1 in which the surface of the positive electrode active material particles is not coated with a rare earth element compound, an increase in polarization resistance due to gas generation due to electrolytic solution oxidative decomposition at the positive electrode accompanying charge / discharge and deposition of decomposition products, While the high-voltage and high-temperature cycle characteristics are poor and the battery thickness is increased due to the deterioration of the positive electrode material due to the dissolution of the positive electrode active material, the comparative example 1 has a rare earth element such as erbium. By covering the surface of the positive electrode active material particles with this compound, side reactions on the surface of the positive electrode active material are suppressed, and gas generation and deterioration of the positive electrode material are less likely to occur.

  However, just using a positive electrode active material whose particle surface is coated with a rare earth element compound, that is, positive electrode active material A, as a positive electrode active material is insufficient in improving high voltage and high temperature cycle characteristics and reducing the increase in battery thickness. In comparison between Examples 1 and 10 and Comparative Example 1, the surface of the positive electrode active material particles was coated with a rare earth element compound, and a small amount of lithium molybdate or lithium tungstate was added to the positive electrode active material mixture slurry. It can be seen that the effect of improving the high-voltage and high-temperature cycle characteristics and the effect of reducing the increase in the battery thickness becomes remarkable by mixing them together.

  This remarkable improvement in the high-voltage and high-temperature cycle characteristics and the effect of reducing the increase in battery thickness are considered to be due to the following mechanism. That is, by covering the surface of the positive electrode active material particles with the rare earth element compound, the deterioration of the positive electrode material is suppressed, but as it is, the crystalline structure accompanying the deposition or charging / discharging of the electrolyte reductive decomposition product on the negative electrode is maintained. Since the material deterioration of the negative electrode due to brittle fracture or the like proceeds at a constant speed, the deterioration of the negative electrode material is relatively faster than the deterioration of the positive electrode material.

  Therefore, when the charge capacity of the positive electrode exceeds the charge capacity of the negative electrode, lithium metal begins to deposit on the negative electrode, but since the deposited lithium is very reactive, the reductive decomposition of the electrolytic solution is accelerated, and as a result As the battery capacity deteriorates, battery swelling increases.

  On the other hand, lithium molybdate or lithium tungstate is a chargeable / dischargeable material and functions as a positive electrode active material. However, the transition metal easily dissolves with charge / discharge, and cycle deterioration is quick. However, the dissolved metal component does not deteriorate the battery characteristics of the negative electrode.

  Here, as the positive electrode active material, in addition to the positive electrode active material A whose particle surface is coated with a rare earth element compound, a small amount of the lithium molybdate, lithium tungstate, or the like is added as the positive electrode active material B, whereby the positive electrode material becomes Since it deteriorates moderately, the balance between the deterioration rate of the positive electrode material and the deterioration rate of the negative electrode material is improved, and the high voltage and high temperature cycle characteristics can be improved while sufficiently suppressing the oxidative decomposition of the electrolyte. Become.

  That is, in order to improve the high-voltage high-temperature cycle characteristics and reduce the increase in battery thickness, it is necessary to cause the material deterioration of the positive electrode and the negative electrode to progress to the same extent. If the amount of lithium molybdate or lithium tungstate contained in the catalyst is too small (Comparative Example 3), the cathode material does not deteriorate, and if too much (Comparative Example 4), the cathode material deteriorates more than necessary. End up.

  From the results of Comparative Examples 3 and 4, the amount of lithium molybdate or lithium tungstate contained in the positive electrode active material mixture as the positive electrode active material B is 0.005% by mass or less with respect to the positive electrode active material A, Or when it is 5.0 mass% or more, it turns out that the high voltage high temperature cycling characteristics improvement effect and the battery thickness increase amount reduction effect are not sufficiently obtained.

  From the results of Example 8, if the amount of lithium molybdate or lithium tungstate contained in the positive electrode active material mixture as the positive electrode active material B is 0.01% by mass or more with respect to the positive electrode active material A, It can be seen that an improvement in the high voltage and high temperature cycle characteristics and a reduction in the increase in battery thickness can be obtained sufficiently. On the other hand, from the results of Example 9 and Comparative Example 4, the upper limit of the amount of lithium molybdate or lithium tungstate to be contained in the positive electrode active material mixture as the positive electrode active material B may be 2.0% by mass or less. Preferably, 1.0 mass% or less is more preferable.

  In addition, since it is well known that rare earth elements have similar chemical or physical characteristics, various rare earth element compounds are used as the compounds covering the particle surface of the positive electrode active material A in the present invention. It is possible. This is supported by the fact that the effects of the present invention are exhibited in Examples 2 to 5 as in the case of the erbium compound even when the ytterbium compound, terbium compound, holmium compound, and lutetium compound are used. It is done.

  Other rare earth elements include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy), thulium ( Tm).

  Further, from the results of Examples 6 and 7, the adhesion amount of the rare earth element covering the particle surface of the positive electrode active material A was at least 0. 0 with respect to lithium cobaltate as the active material of the positive electrode active material A. It can be seen that the effect of the present invention is surely achieved when the content is 01 mol% or more and at most 0.3 mol% or less.

  Further, as shown by the results of Example 11 and Comparative Example 6, even when nickel, cobalt, lithium manganate is used as the active material of the positive electrode active material A, the effect of the present invention is achieved. As the active material of the positive electrode active material A, it is speculated that the effects of the present invention can be obtained even when other lithium cobalt composite oxides or different metal element-added lithium cobalt composite oxides are used.

  In Comparative Example 5 in which molybdenum was added when the positive electrode active material was fired, the high-voltage and high-temperature cycle characteristics deteriorated compared to Comparative Example 1, and the decrease in the increase in battery thickness was hardly observed. This is presumably due to the fact that the ionic radius of molybdenum atoms is much larger than that of cobalt atoms, so that molybdenum does not dissolve in the resulting positive electrode active material and segregates on the surface of the positive electrode active material particles. That is, in Comparative Example 5, since lithium molybdate is hardly present in the positive electrode active material mixture, the segregated molybdenum is selectively dissolved during the cycle in addition to the effects of the present invention. Therefore, the grain boundary resistance increases and the battery operating voltage is greatly reduced.

  Therefore, in order to appropriately include lithium molybdate or lithium tungstate as the positive electrode active material B in the positive electrode active material mixture, the method of adding molybdenum or tungsten at the time of firing the positive electrode active material is not preferable and prepared separately. It can be seen that the positive electrode plate is preferably prepared by mixing the positive electrode active material A and the positive electrode active material B at the time of preparing the positive electrode active material mixture slurry.

Claims (4)

In a nonaqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and a nonaqueous electrolyte,
The positive electrode active material is
A positive electrode that is at least one of lithium cobaltate and nickel / cobalt / lithium manganate and has fine particles of at least one of a rare earth element hydroxide and an oxyhydroxide on the particle surface. Active material A,
A positive electrode active material B comprising at least one of lithium molybdate and lithium tungstate,
The content of the positive electrode active material B is Ri the positive active material der 2.0 wt% or less than 0.01 wt% relative to A,
The non-aqueous electrolyte secondary battery, wherein a charge potential of the positive electrode is 4.35 V or more and 4.6 V or less on the basis of lithium .   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein an average particle diameter of the fine particles made of at least one of the rare earth element hydroxide and the oxyhydroxide is 100 nm or less.   The amount of fine particles comprising at least one of the rare earth element hydroxide and oxyhydroxide in the positive electrode active material A is at least one of the lithium cobalt oxide and nickel / cobalt / manganese lithium manganate. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content is 0.01 mol% or more and 0.3 mol% or less. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is made of graphite .